The present invention relates to a method and apparatus for treating pulp. Preferably the method and apparatus according to the invention are applicable for thickening fiber suspensions of the wood processing industry. The method and apparatus according to the invention are especially preferably suited for applications where liquid is to be removed from fiber suspensions with a relatively low energy consumption, whereby the most obvious applications are pre-thickeners or the like used in connection with various known filters. However, the thickener according to the invention may in some applications be utilized as the actual filter, by means of which consistencies in the range of up to 15% may be obtained.
Traditionally, fiber suspensions have been screened at a consistency of about 1-2% in connection with chemical and other pulping. Fiber suspensions, i.e. pulp, are easily screened at this consistency, the result being a good purity level of the pulp. After screening the pulp has been thickened normally with suction drum or disc filters to a consistency of about 8-16%. This technology is as such quite serviceable, but low screening consistency increases the costs of pumping and the suction drum and disc filters require a large building volume.
With new technology, screening departments have been introduced in which a screening department feed pump creates a pressure difference, by means of which the pulp is conveyed through the screens and further by means of super-atmosphere pressure in the screens into a closed hydraulic filter. Said technology is described in Patent application EP-A-0390403. The advantage of the process described in said publication is that expensive, space-consuming suction drum and disc filters are not needed. A disadvantage of the described process is that the screening consistency has had to be raised to a range of 3-5%, which in its turn has caused problems in running and sometimes also pulp impurity problems. The operation of closed hydraulic filters has required a feed consistency of at least 3-5%, which has restricted the possibilities of choosing the screening consistency freely.
An objective of the present invention is to enable the building and running of screening departments so that the consistency in the screening department is arranged to be optimal in view of screening, whereby the consistency of the actual screening is lower than the feeding consistency of the filter whereto the pulp is finally fed. This invention enables the screening to be carried out at a low consistency and still use new efficient closed hydraulic filters. The typical screening consistencies are 2-4% and the typical feeding consistencies of the filter are 3-6%. Thus, the difference in consistency between screening and filter feed is typically 1-3%, mostly 1-2%. Further, it is to be noted that sometimes e.g. the discharge consistency of a process tower and/or apparatus etc. adjacent to it limit the consistency to be too low in view of the subsequent process stage, whereby it is necessary to raise the consistency of the pulp to be appropriate for the subsequent process stage.
The consistency difference between screening and filtering is created using a pre-thickener as shown in FIG. 1 prior to the actual filter. The pre-thickener is preferably pressurized and hydraulically filled with liquid. In that case, the whole screening department, comprising screens, a pre-thickener and the actual filter, operates in a closed space, whereby the amount of odor compounds released into air remains small. The screening consistency is 2-4%, the consistency after the pre-thickener 3-6% and the consistency after the main filter 8-40%, preferably 10-16% when the filter is a washer-type filter and 25-40% when the filter is a press-type filter.
Hydraulic thickeners suitable for increasing the consistency of pulp have been presented earlier. Patent application EP-A-0 298 499 discloses one thickener solution, by means of which the consistency of fiber suspension may be raised from the feeding consistency of 0.3-1.0% to a range of 1.0-5.0% or from the feeding consistency of 3-10% to a range of 10-25%. Thus, it is quite an efficient thickener capable of producing major changes in consistency. This apparatus is, however, too expensive and its operating costs, mainly the energy consumption, make it in practice unpractical e.g. for the present purpose.
Literature discloses simple thickeners consisting of only a perforated tube in which the pulp flows. Such thickeners have been described e.g. in patent publications EP-B-0274690 and SE-C-227590. However, practice has shown that devices as simple as these are not suited for industrial use. Their problem is that although they do operate temporarily, their filter surfaces tend to get clogged periodically and their re-opening or keeping them clean tends to be unsuccessful if they do not have a rotor of some kind. Thus, in connection with the present invention, it has been decided to use an apparatus of another kind, in spite of the fact that, at its lowest, the demand for consistency increase is in the order of one percentage unit, even though in some special circumstances the apparatus may be used to raise the consistency as much as near to 10%. Usually in that case, however, the situation is that the initial consistency of the pulp is already at a relatively high level, at about between 7-10 percent. To put it more exactly, the apparatus is at its best when the aim is to raise the consistency of the pulp in the apparatus about two-fold. However, it is naturally possible to reach other kinds of changes in the consistency by adjusting the operation of the apparatus. In the present case, the tendency of the filtering surface to get clogged is increased by pressure pulses occurring both in the screening department and its devices, which tend to force fibers into the slots of the filter surface, which in its turn results in the clogging of the filter surface, if a filter surface cleaning means is not used.
Prior art knows also an apparatus according to U.S. Pat. No. 4,085.050, functioning as a continuously operating filter, which apparatus comprises a vertically arranged cylindrical filter surface, a filtrate chamber arranged outside the filter surface, a centrally open screw thread arranged inside the filter surface and a feed conduit for the material to be filtered and a discharge conduit for the thickened material arranged at the upper and lower ends of the filter surface respectively. The apparatus functions so that a so-called precoat acting as the actual filtering material forms or alternatively is formed on the filter surface. As the material to be filtered is precipitated on this precoat, the screw thread wipes the precipitated layer off letting new material to be precipitated on the precoat layer. Said precoat layer is cleaned by feeding washing liquid through the shaft of the apparatus, which pressurized washing liquid cleans the precoat layer.
U.S. Pat. No. 4.464,253 describes an apparatus wherein the dry solids content is raised high and the consistent part is discharged via a cone. This kind of procedure is not possible with fiber suspension, because fiber suspension, being consistent, will not flow in a convergent cone. Said patent teaches that the pressure difference required in the filtering process is created by means of the feed pump of the apparatus alone or by means of said feed pump and a vacuum arranged in the filtrate compartments together. The apparatus of this patent is meant to be used so that the material to be filtered is fed into the upper end and the thickened material is discharged from the lower end of the apparatus. The apparatus comprises cylindrical and conical parts and is most obviously meant for high contents of residual dry matter.
Further, U.S. Pat. No. 5,034,128 deals with a similar kind of apparatus for raising the consistency to a range of 5-30% from a low initial feeding consistency. In this case it is an apparatus, which is specially meant for removing liquid from fiber suspensions of the pulp industry, but the goal is a high increase in consistency and a high final consistency. A characteristic feature of the apparatus is that the screw is closed, i.e. the screw thread is fastened directly to a cylindrical or conical shaft core. The apparatus is further characterized in that the screw thread is arranged so close to the filter surface that it keeps the filter surface clean. In other words, the apparatus functions without a precoat layer. It is our conception, however, that the apparatus cannot function in the way described in the publication, but when pursuing high consistencies, the screw of the apparatus has to be used like a press.
U.S. Pat. No. 4,582,568 deals with yet another apparatus used in order to thicken fiber suspension by means of a screw press. However, a characteristic feature of this apparatus, unlike a few above-mentioned devices, is that the pressure difference required for the thickening is generated by the screw of the apparatus itself. Said patent publication deals with a combination of a thickener and a screw press, the thickener being meant for raising the consistency of the fiber suspension to correspond to the feeding consistency of the screw press. The function of the thickener is carried out by an apparatus provided with a closed screw surrounded in a small clearance with a filter surface. The fiber suspension is fed into the inlet end of the screw, wherefrom the screw further pushes the suspension against a hydrostatic pressure created by an upward directed discharge conduit arranged at the discharge end of the screw. A problem of the screw thickener described in said patent is that the screw is closed, whereby, as the apparatus stops, the flow of fiber suspension through the apparatus will also stop completely. Another problem is that the operational efficiency of the filter surface is relatively low, because the filter surface functions actively only in the vicinity of the inlet end. This is due to the characteristic feature of the closed screw that it feeds the pulp inside it as an essentially plug-like flow, whereby only the pulp layer facing the filter surface is efficiently thickened, the rest of the pulp passing nearer to the shaft of the screw without being essentially thickened. Liquid is filtrated to the filter surface only through a thickened pulp cake formed on the filter surface and the thickening rate is slow. This results in a highly limited capacity of the device, and raising the capacity is not easy, either, because the problem of the closed screw can only be eliminated by increasing the dimensions of the device.
The prior art apparatuses described above have some disadvantages of which at least the following are worth mentioning:
in case of an essentially atmospheric xe2x80x9cdownstream flowingxe2x80x9d apparatus provided with an open screw thread (U.S. Pat. No. 4,085,050) the adjustment of the apparatus for cellulose i.e. pulp is difficult. Moving the pulp downwards so that it could be essentially thickened at a consistency of less than 8% is not possible due to the characteristics of the stock.
in our opinion, a device provided with a closed screw does not function with dilute pulp, i.e. at a consistency of 1-5%, because at the moment when the pulp is fed in under pressure, a flow revolving spirally along the screw thread is immediately generated which flushes off the cake collected onto the filter surface, thus hampering the thickening. If the inlet pressure is very low, the thickening carried out by the apparatus starts well, but when there is a layer of thickened pulp on the filter surface, the thickening is essentially decelerated due to reasons described above in connection with the U.S. Pat. 4,582,568. In addition to that, a device provided with a closed screw causes the whole process to stop e.g. in case of actuator breakdown or the like, because even with pulp of low consistency, the thickening of the pulp in the device takes place relatively quickly so that it forms a strong unmovable plug extending throughout the whole diameter of the device.
The apparatus for treating pulp according to the present invention eliminates e.g. said problems of prior art devices. Characteristic features of the apparatus according to the invention are, e.g., that
according to one embodiment of the invention, pulp is fed from the screens into the apparatus through a closed line preferably utilizing the discharge pressure of the screens as the feeding pressure,
according to one preferred embodiment, the feeding consistency into the apparatus is 2-4%, preferably 2-3%,
by means of an apparatus according to one preferred embodiment, the consistency is raised by 1-4%, preferably by 1-2%,
the discharge consistency utilizing an apparatus according to one preferred embodiment is 3-6%, preferably 4-6%,
more generally speaking, the feeding consistency of the apparatus may vary in between about 0.8 and 8 percent, and the discharge consistency, in its turn, may be regulated to between about one and 15 percent,
the apparatus according to one preferred embodiment of the invention is coupled between the pressure screen and the filter, whereby it functions so that when the pressure of the pulp in the screen raises above atmospheric pressure, the pre-thickener is pressurized, too, and the pressure prevailing in the screen pushes the filtrate through the filter surface of the pre-thickener,
the pressure prevailing in an apparatus according to one preferred embodiment of the invention is preferably high enough to feed the pulp into the filter located after the pre-thickener,
when the apparatus according to one preferred embodiment of the invention is pressurized, the apparatus may be mounted in any position. Thus, e.g. when the apparatus is mounted vertically, the inlet end may be arranged either at the lower or the upper end of the apparatus. And, consequently, the discharge end may be located either at the upper or the lower end,
it is characteristic of the apparatus according to the invention that fresh pulp is delivered onto the whole length of the filter surface. The filter surface is constantly wiped by one or several screw threads which collect/s to their/its leading side the pulp thickened onto the filter surface and leave to the back side, i.e. their trailing side a cleaned filter surface, onto which fresh pulp flows through the center of the open screw.
Other characteristic features of the method and apparatus according to the invention are disclosed in the appended patent claims.